1. Field of the Invention
This invention relates to measuring systems for three-dimensional objects. In particular, this invention relates to an improved method of and apparatus for making the measurement of the diameter and circumference of three-dimensional objects which are easily deformable, wherein normal dimensions are difficult to maintain during measurement.
2. Description of Related Art
An air bag inflator for use in a driver side air bag module or a passenger side air bag module comprises an outer rigid housing, a filter unit containing filter screens and filter material, gas generating material, an inner housing, and igniter material. Screen elements, which are flexible units having a circular cross section, are normally manufactured by rolling individual units of screen cloth around a mandrel and spot welding the screen at a number of locations in order that the screen unit retains its circular cross section. Herein, flexible is defined as a semirigid object which is readily deformed from its normal shape by the application of light compressive force. These screen elements must be manufactured to within close tolerances to provide for proper assembly of the air bag inflator unit. Sample screen elements are tested periodically to verify conformance of the diameter and circumference of the screen elements to within specified tolerances.
Diameter and circumference measurements of the screen units must be done by means which do not distort the flexible screen elements in order to obtain the correct measurements. Also, the measuring device should be capable of correcting for screen elements which have been distorted from their normal shape due to handling. Therefore, conventional measuring devices such as calipers or the like cannot be used due to possible deformation of the screen element resulting in incorrect measurements.
Solutions have been suggested for the measurement of rigid cylindrical objects. Golinelli et al., U.S. Pat. No. 4,543,725; Vreeland, U.S. Pat. No. 3,303,572; and Geiler et al., U.S.S.R. Inventor's Certificate (11) 521451, disclose devices for checking the external diametrical dimensions of rigid circular parts. Golinelli utilizes a support means and two feelers which move with respect to the support. A transducer provides an electrical signal proportional to the position of the feelers. Vreeland utilizes two probes mounted 90.degree. from one another and connected by a spring member. As the diameter is measured, the spring member expands or contracts and the strain is measured and converted into diametrical measurements. Geiler discloses a gauge with three probes attached to two cylindrical half-rings compressed onto the object by twin springs. The gauge is held to the object to be measured by three axial stops. Measurement is taken by two fixed measuring probes and one sensitive measuring probe. These apparatus rely on measurement of cylindrical objects by probes contacting the object at a fixed number of points. Further, these apparatus do not readily compensate for objects which have an elliptical cross section.
Another measuring device known as a pi tape can be used to measure circumference. This apparatus has the advantage of accommodating the deformation of the object to be measured, but does not compensate for elliptical error. Further, this device is difficult to use and is prone to operator error due to variability in interpolation of the reading required of the operator to determine the final measurement. Alternative measurement systems include visual identification systems and laser gauging systems. These systems are accurate and highly reproducible. They are costly, however, and sometimes slow in operation.